Aerospace thermoplastic composites are relatively difficult to process because the resins contain significant amounts of solvent and cure at relatively high temperatures often with a limited range of temperature between the boiling point of the solvent, melting point of the resin, and curing temperature of the resin. We call this temperature range the processing window with conventional autoclave processing where the prepreg laminate is enclosed within vacuum bags and heated within a pressurized oven, it is often difficult to obtain substantially fully consolidated products. Operating in the narrow processing window is difficult, but doing so is essential to evaporate the solvent, to melt the resin so that plies in the laminate will consolidate and flow, and to cure the resin by its chain extension condensation reaction. Augmenting the processing with ultrasonic vibration to supplement the conventional practice of pressing the melted material for momentum transport ("flow") should improve the products while reducing the cure cycle. Therefore, the process of the present invention saves time and reduces waste or rework. Since the resins cost over $100 per pound and the manufacturing process is relatively slow and labor intensive, the present process promises a significant economic benefit.
In U.S Pat. No. 4,288,398, Lemelson described alternative methods for controlling the internal structure of molded or extruded plastics or metals. Lemelson suggested using ultrasound alone or in combination with other forms of energy to orient the grain or crystalline structure. Lemelson introduced ultrasound to the melted material during its consolidation to control the internal structure. The process of the present invention uses ultrasound to assist momentum transport after heating the resin to its softening or melting temperature and during the pressure application for resin flow phase of its consolidation.
Resins that cure to high operating temperature thermoplastic composites generally require high processing temperatures. For a resin-fiber composite system capable of operating at 425.degree. F. or higher, the resin must have a glass transition temperature (T.sub.g) of 525.degree. F. after equilibration with the operating environment and an "as processed" T.sub.g approaching 600.degree. F. A resin with a high T.sub.g will also have a high melting (T.sub.m) or softening (T.sub.s) temperature. The temperature differential between the T.sub.g and the T.sub.s is established by the molecular weight distribution and usually is on the order of 200.degree. F. In addition, the viscosity of such a high melting resin above the melt or softening temperature will likely be greater than 10.sup.6 Pa.cndot.sec. Therefore, consolidating acceptable quality laminates using these resins requires high pressures and temperatures.
To date, only small parts that will fit within the platens of a press could be fabricated using extremely high T.sub.g resins. Processing in this manner requires matched tooling and has been limited to high value parts such as engine components.
Attempts to consolidate large planform area parts (such as exterior skin panels for composite aircraft) in an autoclave have been unsuccessful. The laminates exhibited extensive porosity and suffered from microcracking because of the volatiles and by-product gases generated during the condensation reaction of the resin when it cured. High viscosity of these high melting resins inhibited momentum transport and resin flow during the pressurized portion of the autoclave cycle. While processing might be possible at even higher temperatures and pressures, conventional autoclaves are not designed for the increased pressures. Replacing conventional autoclaves to allow higher pressure operation is too expensive to justify using the high melting resins available today for today's applications.